This invention relates to the field of ophthalmic progressive multifocal lenses. In particular, it relates to a new progressive lens design having both a short progressive corridor and a progressive geometry compensated for the unwanted effects of the short corridor, making the lens especially suitable for use in small eyeglass frames.
It is appropriate to being by defining the corridor length of a progressive lens. In general, the term refers to the vertical drop from the distance fitting center to the top or beginning of the reading portion. But precisely where does the corridor end and the reading portion begin? According to the ANSI Z80.1-1995 American National Standard for Prescription Ophthalmic Lenses, the tolerance on addition power for multifocal and progressive addition lenses is .+-.0.12 D for additions up to 4.00 D. Thus, as illustrated in FIG. 1, it is reasonable to define the corridor length of a progressive lens as the vertical drop from the distance fitting center (represented throughout all drawings by the symbol ##STR1## to the point of the corridor where the nominal add power, less 1/8 D (0.125 D), is attained.
By this definition, the corridor length of the average commercially-available progressive lens is about 17 mm. According to the manufacturers, these lenses generally require a 22 mm frame depth below the distance fitting center. Provided that the frame depth requirement is met, a lens having a 17 mm corridor can be expected to provide good near vision utility. Moreover, the associated gradients of power and astigmatism in the progressive surface of a lens having a 17 mm corridor are usually weak enough to ensure good overall visual comfort.
The 22 mm frame depth required for the typical progressive places a lower bound on the vertical or B dimension of the frame in which the lens is mounted. For example, as depicted in FIG. 2, if the fitting center is positioned 4 mm above the equator of the frame, then to ensure sufficient reading utility, the B dimension of the frame can be no less than 2.times.(22-4)=36 mm. In recent years, however, frame sizes with B&lt;36 mm have become increasingly popular throughout the world. In Europe and Asia, for example, the B dimension of the average frame is currently 30-35 mm and 25 mm is not uncommon. Lenses having a 17 mm corridor and requiring a 22 mm frame depth generally cannot be used in such small frames without sacrificing the reading function of the lens. For example, as illustrated in FIG. 3, if the fitting center of a lens having a 17 mm corridor is placed 4 mm above the equator of a frame having a B dimension of 30 mm, then the drop from the fitting center to the lower inside edge of the frame will be 30/2+4=19 mm. Thus, vertically only about 2 mm of prescribed reading power lies within the boundary of the frame. This is inadequate for all but the most perfunctory near-vision tasks.
Thus, the need exists to design a progressive lens specifically for frames having a B dimension less than 36 mm. At first sight this seems to be a simple task. Suppose that one wants to design a lens having a 13 mm corridor, 4 mm shorter than the current average. A design meeting this requirement might seem to be readily obtained simply by scaling down the layout of a conventional long-corridor design by a factor 13/17=0.76, while maintaining the same add power. Unfortunately, this simple design procedure also increases the surface power and astigmatism gradients of the scaled-down lens by a factor 17/13=1.31, an increase of 31%. The distortion effects generated by these elevated gradients may not be tolerated by most wearers of the lens. That, in fact, is why progressive lenses today are designed with longer corridors. To illustrate, FIG. 4 shows the surface mean power and astigmatism contour plots for a conventional prior art progressive having a relatively short corridor (15.5 mm) and overlayed by a frame of B=30 mm. The astigmatism plot reveals particularly strong gradients of astigmatism throughout most of the lens and elevated levels of astigmatism (2.5 D), all resulting from the shortness of the corridor. In other words, one cannot achieve an acceptable short corridor lens by just shortening the corridor of a long corridor lens.